The keys of `self.files` are url encoded. This leads to errors when the path of a file contains special characters as `src_path` is not url encoded.

Modifications:

- url encode the directory path
- generate `self.files` in the `on_files()` function instead of `on_page_markdown()` (this is only an optimization, not strictly needed)
- test (directory with special characters)
---------

Co-authored-by: Benjamin Graf <[email protected]>

The instructions didn't get you a working environment for testing.
They also were missing a number checks that happen in CI. This
now documents everything you need to run locally to get CI to pass
except for the "running on different platforms". All the Ubuntu checks
seems mostly as "safety" but the Windows checks can discover bugs that
don't occur on Linux. I haven't documented that at this time as it
was outside of the scope of what I wanted to accomplish with this change.

This commit fixes naive handling of relative links when proofing. The
problem here was the assumption that a relative link of foo.html that
was coming from a file at bar/fubar.md in the filesystem would always
appear at bar/foo.html.

That is not always true. If directory_urls was on, then it wasn't true.
Thus, #46.

It was also not true when using relative links between blog posts when
using the mkdocs material blog plugin.

This commit changes the relative url lookup logic to be more robust and
in the process make the proofer work with both the mkdocs material blog
plugin and with directory_urls on.

It does this by storing additional information in the lookup files that
we use to find information about pages. We can now look up a file by its
src_uri entry, for example blog/posts/index.md and then use that to get
the dest_uri like blog/12/10/some-post/ and then use that dest_uri value
to robustly handle figuring out where a relative link is pointing.

Closes #46
Closes #70